Stroboscopic device with electric shock preventive circuit

ABSTRACT

In a stroboscopic device of the type including a trigger circuit which is charged by a power source, a trigger circuit for flashing a flash discharge tube when sync contacts are closed, and a terminal connected to the sync contacts and to first and second terminals of the trigger capacitor, unidirectional current control means is inserted between one of the contacts connected to said first terminal of the trigger capacitor and one terminal of the power source so as to interrupt the short circuit current when the contact connected to said second terminal of the trigger capacitor and the terminal of the power source are short circuited.

United States Patent 1191 Nakamura Nov. 18, 1975 STROBOSCOPIC DEVICE WITH ELECTRIC SHOCK PREVENTIVE CIRCUIT [75] Inventor: Zenzo Nakamura, Urawa, Japan [73] Assignee: Canon Kabushiki Kaisha, Tokyo,

Japan [22] Filed: Dec. 23, 1974 [21] Appl. N0.: 535,818

[30] Foreign Application Priority Data Dec. 27, 1973 Japan 48-4238 [52] US. Cl. 315/241 P; 317/33 R; 317/33 SC; 317/52; 317/262 R; 331/63; 328/7 [51] Int. Cl. H05B 41/32 [58] Field of Search 315/241 P; 328/7; 330/207 P; 317/52, 262 R, 39, 43, 33; 331/63 [56] References Cited UNITED STATES PATENTS 3,569,779 3/1971 Luursema 315/241 P Primary Examiner-John Kominski Attorney, Agent, or Firm-William R. Woodward [5 7] ABSTRACT In a stroboscopic device of the type including a trigger circuit which is charged by a power source, a trigger circuit for flashing a flash discharge tube when sync contacts are closed, and a terminal connected to the sync contacts and to first and second terminals of the trigger capacitor, unidirectional current control means is inserted between one of the contacts connected to said first terminal of the trigger capacitor and one terminal of the power source so as to interrupt the short circuit current when the contact connected to said second terminal of the trigger capacitor and the terminal of the power source are short circuited.

4 Claims, 5 Drawing Figures T1 b i Sheet 1 of 2 3,921,034

U.S.- Patent Nov. 18, 1975 FIG. I

j l I FIG. 2

U.S. Patent Nov. 18, 1975 Sheet2 of2 3,921,034

STROBOSCOPIC DEVICE WITH ELECTRIC SHOCK PREVENTIVE CIRCUIT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric 'shock preventive circuit employing unidirectional current control means for use with a stroboscopic device.

2. Description of the Prior Art In the prior art stroboscopic devices, the energy stored in a main capacitor is discharged in response to the sync flash signal from a camera main body to flash a discharge flash tube such as a Xenon filled discharge tube. However, when one touches mistakingly the contacts for connection with the so-called sync tenninal or contacts and the terminals of the power source of the stroboscopic device, the energy stored in a trigger capacitor in a trigger circuit discharges through the sync contact, the human body, and the terminal of the power source, thus giving an electric shock to the operator.

In view of the above, the present invention has for its object to overcome the above electric shock problem encountered in the prior art stroboscopic devices. Briefly stated, according to the present invention, a unidirectional current control means is inserted between the 'sync connection terminal or Contact and the terminal of the power source, thereby interrupting the short circuit current which gives rise to the electric shock. The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING unidirectional current control means thereof; and

FIG. is a circuit diagram of a third embodiment of the present invention including an electric shock preventive circuit employing a thyristor as a unidirectional current control means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment, FIG. 1

Referring to FIG. 1, a power source 1 withterminals 1a and lb is connected through a DC-DC converter 2 in parallel with a flash discharge tube 3, a trigger circuit 4 and a main capacitor 5. The power source 1 is shunted by a capacitor 6, and is provided with a power switch 7. The converter 2 comprises a transistor 8, a resistor 9, a capacitor 10, a step-up transformer 11, and

. a diode 12. The emitter of the transistor 8 isconnected through the switch 7 to the positive terminal of the .power source 1 while the collector is connected through the primary winding P of the transformer 11 to the negative terminal of the power source 1. The base of the transistor 8 is connected through the resistor 9 to the negative terminal of the power source 1 and to the positive terminal thereof through the feedback winding F of the transistor 11, the capacitor 10 and the switch 7. The base of the transistor 8 is also connected through the secondarys of the step-up transformer 11 and a forward connected diode 12 to the anode of the main capacitor 5. The cathode of the main capacitor 5 is connected to the junction between the capacitor 10 and the switch 7 through a forward connecteddiode 13 which is a unidirectional current control means. The trigger circuit 4 comprises a resistor 14, a capacitor 15, and a trigger transformer 16. The anode of the capacitor 5 is connected in series to the resistor 14, the capacitor 15, and the primary winding of the trigger transformer 16 in the order named. The secondary winding of the trigger transformer 16 is connected to the trigger terminal of the flash discharge tube 3. A terminal 17 is connected to a sync terminal of a camera. One contact 17a is connected to the junction between the resistor 14 and the capacitor 15 while the other contact is connected to the junction between the primary winding of the transformer 16 and the diode 13. As indicated by the broken lines, the diode 13 may be inserted between the main capacitor 5 and the primary winding of the transformer 16 as indicated by the reference numeral 18.

Next the mode of the operation of the first embodiment with the above construction will be described. When the power switch 7 is closed, the base current of the transistorflows through the resistor 9 so that the collector current flows through the primary winding of the transformer 11. The voltage is induced across the feedback winding P of the transformer 11 so that the base current of the transistor 8 is increased. As a result, the collector current is also increased. Then the magnetic flux of the transformer 11 is saturated so that the counter electromotive force is induced. Consequently, the voltage with the polarity opposite to that of the counter electromotive force is induced across the feedback winding F. Therefore the transistor 8 is turned off. However the switch 7 remains closed so that the base current of the transistor 8 immediately starts to flow again. Therefore the above described operation is cycled again. The current flowing through the primary winding P induces the voltage across the secondary S so that both the main capacitor 5 and the trigger capacitor 15 will be charged. When the contacts 170 and 17b of the sync terminal 17 are closed after the main capacitor 5 and the trigger capacitor 15 have been charged, the trigger capacitor 15 discharges so that the current flows through the primary winding of the trigger transformer 16, inducing the voltage across the secondary thereof. As a result, the flash discharge tube 3 is triggered and is flashed by the discharge of the main capacitor 5.

With the stroboscopic device of the type described, when one touches the contact 17a of the terminal 17 and the positive terminal In of the power source 1 when both the main and trigger capacitors 5 and 15 have been charged to a peak voltage, the trigger capacitor 15 is discharged through a circuit including the contact 170, the hand, the positive terminal E of the power source 1, the switch 7, and the trigger transformer 16 if the diode 13 is not provided, thus causing an electric shock. However, according to the present invention, the diode 13 or 18 is inserted so that the discharge Current from the trigger capacitor is interrupted. Thus-an electric shock may be prevented.

In the first embodiment, the unidirectional currentcontrol means has been described as consisting of one diode 13 or 18, but a plurality of series-connected di-.

In'the second embodiment shown in FIG. 3, reference numeral denotes a power source with terminals 20a and 20b; 21, a DC-DC converter comprising transistors 22 and 23, resistors 24 and 25, a capacitor 26, a step-up transformer 27, and a rectifier connected to the secondary of the transformer 27', 29, a power switch", 30, a main capacitor connected through the recitifler 28 to the secondary of the step-up transformer'27 for storing the energy for flashing a flash discharge tube 31 connected in parallel to the main capacitor 30; and 32, a trigger circuit for triggering the flash discharge tube 31 and comprising a capacitor 33, resistors 34 and 35, and a trigger transformer 36. A terminal 37 is connected to a sync terminal of the camera, and

its one contact 37:; is connected to the anode A of the trigger capacitor 33 while the other contact 371) is connected to the cathode B thereof. An electric shock preventive transistor 38 with a bias resistor 39 is inserted as a unidirectional current control means.

Next the mode o'foperation of the second embodi ment with the above construction will be described hereinafter. When the power or main switch 29 is closed, the base current flows from the transistor 22 through the feedback winding F of the transformer 27 and the resistor so that the collector current flows through the primary winding P, of the transformer 27. When the magnetic flux in the transformer 27 reaches a saturation point, the counter electromotive force is induced across the primary Winding P so that the counter electromotive force is also induced across the feedback winding F As a result, the transistor 22 is turned OFF while the transistor 23 is turned ON because the voltage is induced across the feedback winding F so that the collector current flows through the primary winding P When the magnetic flux reaches a saturation point again in the transformer 27, the counter electromotive force is induced across the feedback winding F so that the transistor 23 is turned OFF. Since the counter electromotive force is also induced across the feedback winding F,, the transistor 22 is turned ON. Thereafter the above operation will be cycled.

When current flows through the primary windings P and P the counter electromotive force is induced across the secondary S, so that both the main capacitor and the trigger capacitor 33 will be charged. When contacts 37a and 37b of the terminal 37 are closed after the main and trigger capacitors 30 and 33 have been charged, the trigger capacitor 33 discharges to cause the current to flow through the primary winding of the trigger transformer 36. Then the trigger voltage is induced across the secondary of the transformer 36 so that the flash discharge tube 31 is triggered and is flashed by the discharge of the main capacitor 30.

Even if one touches the contact 37a and the terminals 20a and 20b of the power source 20 after the main capacitor 30 and the trigger capacitor 33 have been charged. the discharge current from the trigger capacitor 33 is interrupted by the transistor 38 so that an electric shock may be prevented.

In the second embodiment, the unidirectional current control means has been described as comprising 5 one transistor 38, but it is to be understood that a Darlington circuit consisting of two transistors as shown in FIG. 4 may be employed instead of the transistor 38.

Third Embodiment, FIG. 5

In the third embodiment shown in FIG. 5, a power source 41 with terminals 41a and 41b is connected in parallel to a flash discharge tube 42, a trigger circuit 43, and 'a main capacitor 44. The positive terminal of the power source 41 is connected to the anode of the main capacitor 44 through a series circuit consisting of a resistor 45 and a diode 46 while the negative terminal isconnected to the cathode of the main capacitor 44 through a unidirectional current control means which is a thyristor 47 in the instant embodiment. The gate of the thyristor 47 is connected to the cathode of the main capacitor 44 through a resistor 48 and a diode 49. The trigger circuit 43 comprises a capacitor 50, resistors 51 and 52, and a trigger transformer 53. A terminal 54 which will be connected to a sync terminal of a camera has its one contact 54a connected to the anode of the trigger capacitor 50 and the other contact 54b connected to the negative terminal B of the trigger transformer 53. A main or power switch 55 is inserted between the power source 41 and the thyristor 47.

Next the mode of operation of the third embodiment with the above construction will be described hereinafter. When the main switch 55 is closed, both the main capacitor 44 and the trigger capacitor 50 are charged. When the contacts 54a and 54b are closed after the main and trigger capacitors 44 and 50 have been charged, the trigger capacitor 50 discharges so that the current flows through the primary winding of the trigger transformer 53. The trigger voltage is therefore induced across the transformer 53 so that the flash discharge tube 42 is triggered and is flashed by the discharge of the main capacitor 44.

Even if one touches the contact 37a and the terminals 41a and 41b of the power source 41 after the main and trigger capacitors 44 and 50 have been charged,

the discharge current from the trigger capacitor is interrupted by the thyristor 47 so that an electric shock may be prevented.

I claim:

1. A stroboscopic device comprising:

a. a power source,

b. a flash discharge tube electrically connected in parallel to said power source,

c. trigger circuit means for triggering said flash discharge tube, said trigger circuit means including a capacitor which is charged by said power source,

d. a pair of sync contacts connected to first and second terminals of said capacitor, said capacitor being actuated or discharged when said pair of sync 2. A stroboscopic device as setforth in claim 1 comprises at least one transistor. wherein said unidirectional current Control means 4. A stroboscopic device as set forth in claim 1 comprises at least one diode. wherein said unidirectional current control means 3. A stroboscopic device as set forth in claim 1 comprises at least one thyristor. wherein said unidirectional current control means 5 

1. A stroboscopic device comprising: a. a power source, b. a flash discharge tube electrically connected in parallel to said power source, c. trigger circuit means for triggering said flash discharge tube, said trigger circuit means including a capacitor which is charged by said power source, d. a pair of sync contacts connected to first and second terminals of said capacitor, said capacitor being actuated or discharged when said pair of sync contacts are closed, and e. unidirection current control means inserted between one of the terminals of said power source and one of said pair of sync contacts for interrupting the short circuit current when the other of said pair of contacts and the other terminal of said power source are short circuited.
 2. A stroboscopic device as set forth in claim 1 wherein said unidirectional current control means comprises at least one diode.
 3. A stroboscopic device as set forth in claim 1 wherein said unidirectional current control means comprises at least one transistor.
 4. A stroboscopic device as set forth in claim 1 wherein said unidirectional current control means comprises at least one thyristor. 